Breaking news out of the White House tonight. President Trump's national security adviser is stepping down. Trump tweeted that Lieutenant General H.R. McMaster will be replaced by former ambassador to the United Nations John Bolton.

NPR White House correspondent Tamara Keith is with us now to talk through this latest development. Hey, Tam.

TAMARA KEITH, BYLINE: Hello.

CHANG: So this isn't a total surprise, right? I mean, in fact, NPR's Tom Bowman had reported McMaster was on his way out. It was just a question of when it would happen.

KEITH: Though we had been led to believe that it would not happen quite so quickly, you're right. McMaster was on the so-called endangered species list of Trump administration officials. And now he is out. The White House says that they'd been talking about this for some time - the president and McMaster - that they mutually agreed that this would happen. But then that the timeline was expedited in part because there was so much speculation about when McMaster would go. They just wanted to get that cloud out of the way. He is planning to stay on until mid-April.

CHANG: And why was that speculation that he was leaving - why was that such - why was that going for such a long time? Can you unpack why it was so long assumed?

KEITH: Yeah, so there were personality clashes between President Trump and McMaster. There was this sense that Tom Bowman, our Pentagon correspondent, talks about that McMaster was a lecturer and that President Trump didn't respond well to sort of professor McMaster. And also, there were just strong policy disagreements on a number of issues. You know, McMaster had a position that was different than the position that President Trump took on the Iran nuclear deal, on reaffirming Article 5 of NATO, the Paris climate accord. All of these things President Trump and McMaster were on opposite sides. And ultimately, the president, you know, made the decision that the president made. I just want to point out that this comes at a time of incredible change in this White House.

CHANG: Right. This is just the latest shakeup among lots of White House advisers. Can just take a moment to remind us how much instability there's been?

KEITH: So just in the last 16 days there have been three major announced departures. You have Gary Cohn, who was top economic adviser to the president announcing that he was resigning over in part a disagreement about tariffs and trade. Then you have Rex Tillerson, the secretary of state, getting fired by tweet. And now you have the national security adviser out. H.R. McMaster lasted a year and a month, which is...

CHANG: Sounds like a long time (laughter).

KEITH: ...Much longer than the first national security adviser, who only lasted less than a month. But it's a remarkable amount of turnover. And just to say that these three men who are leaving - McMaster, Tillerson and Cohn - were often on the same side on these various policy disputes...

CHANG: Ah.

KEITH: ...And not the side that the president took.

CHANG: So McMaster will be replaced by former ambassador to the U.N. John Bolton. He is a regular on Fox News. Is that a trend you're seeing in these personnel changes?

KEITH: Yes. So Gary Cohn is being replaced by Larry Kudlow, who is a personality and regular on CNBC. And the newest member of President Trump's legal team dealing with the Russia investigation is also a regular on Fox News. There is a trend here. The president is choosing people who says things that he likes to hear on television.